1. Field of the Invention
The present invention relates to a resistor used for a variable resistor, a switch and other electronic input devices, and it particularly has an object to provide a resistor capable of minimizing the contact resistance of the surface of the resistor with a sliding contact or contact member, and a method for producing the resistor.
2 Description of the Related Art
A resistor used for a variable resistor, a switch contact or the like is formed as film on a substrate in a prescribed thickness. To produce this resistor, a mixed solution of a thermosetting binder resin, a solvent for dissolving the binder resin, and a conductive filler such as carbon black is applied onto the surface of the substrate by means of screen printing or the like. The solvent is vaporized in a drying process followed by baking to cure the binder resin.
The resistance characteristic of the above-mentioned resistor is determined depending on the quantity of the filler in the binder resin which forms the resistor, and also influenced by the dispersion state of the conductive filler in the binder resin. In resistors formed in the same pattern, the lower the total resistance value is, the larger the content of the filler is. In resistors containing the same amount of conductive filler, the larger the total resistance value is, the higher the dispersity of the filler is in the binder resin. Namely, when the dispersity of the conductive filler is high, current paths are dispersed among conductive fillers to increase the resistance value as the whole. When the part where the conductive filler is collectively aggregated is increased in the binder resin, the current paths are easily formed in the resistor, resulting in a reduction in the total resistance value.
In an electronic input device in which a sliding contact is slid on the surface of the resistor or a contact member is brought into contact therewith, when the total resistance value of the resistor is large, the contact resistance value of the resistor with the sliding contact or contact member is increased by just that much, and the portion of the contact resistance value is consequently added to the resistance value set by the resistor as a large error.
If the total resistance value of the resistor is not set large in the constitution of a small-sized slide type variable resistor having high resolution, for example, the change quantity of resistance value is minimized when the sliding contact is moved by a short distance, and the range between maximum resistance value and minimum resistance value obtained from the variable resistor is also minimized, so that the high resolution cannot be ensured. However, since the contact resistance is increased when the total resistance of the resistor of the variable resistor is set high as above, the ratio of the error portion by the contact resistance to the resistance value set by the movement of the sliding contact is increased to make it difficult to precisely set the correspondence of the moving position of the sliding contact to the resistance value corresponding thereto.
When the total resistance value of the resistor is reversely reduced to reduce the contact resistance, the range between maximum resistance value and minimum resistance value is too small to obtain sufficient resolution in a small-sized variable resistor.
To solve the conventional problems described above, the present invention has an object to provide a resistor cable of minimizing the contact resistance with a sliding contact or contact member without significantly reducing the total resistance value by making the resistance in the surface of the resistor smaller than the resistance in the inner part thereof, and a method for producing this resistor.
The present invention involves a resistor formed of a conductive resin material comprising a binder resin and a conductive filler mixed to the binder resin, wherein when compared between a surface of the resistor and an internal cross section parallel to the surface of the resistor on the basis of regions partitioned in the same area, dispersity of the conductive filler in the binder resin is lower in the surface than in the cross section.
In this specification, the degree of dispersity of the conductive filler can be defined as follows.
Firstly, when compared on the basis of the partitioned regions, the one having a larger maximum dimension of two or more aggregates of the conductive filler is defined as the one having low dispersity. According to this definition, the maximum dimension of the aggregates of the conductive filler is larger in the surface than in the cross section of the resistor.
Secondarily, when compared on the basis of the partitioned regions, the one having a larger maximum diameter of two or more virtual circles drawable in a part free from the conductive filler is defined as the one having low dispersity. According to this definition, the maximum diameter of the virtual circles is larger in the surface than in the cross section of the resistor.
The method for producing a resistor according to the present invention comprises:
a process for printing, in a prescribed pattern, a mixed solution containing a good solvent with high solubility to a binder resin, a poor solvent lower in solubility than the good solvent and also lower in volatility than the good solvent, a thermosetting binder resin, and a conductive filler;
a process for drying the mixed solution; and
a process for curing the binder resin by baking.
In the mixed use of the good solvent and the poor solvent, the poor solvent is dominant in the surface of the resistor because the good solvent is vaporized first. Therefore, in the baked resistor, the dispersity of the conductive filler can be reduced in the surface to reduce the contact resistance with the sliding contact or contact member. Since the good solvent and the poor solvent are hardly vaporized in the inner part of the resistor, and present therein for a long time, the dispersity of the conductive filler is enhanced. Accordingly, in the baked resistor, the internal resistance can be increased to increase the total resistance value of the resistor.
To that end, a boiling point of the poor solvent is preferably higher than the boiling point of the good solvent, and a difference between both the boiling points is preferably 15xc2x0 C. or higher and 30xc2x0 C. or lower.
In order to make the poor solvent dominant in the surface of the resistor at the time of drying as described above, the drying process is preferably carried out at a temperature higher than the boiling point of the good solvent and lower than the boiling point of the poor solvent.
The good solvent is, for example, at least one or more of dipropylene glycol monomethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, and dipropylene glycol monoethyl ether, and the poor solvent is at least one or more of terpineol, 2-phenoxy ethanol, and 2-benzyloxy ethanol.